Fig.11 The polar aurora--woodcut by Fridtjof Nansen. The lines defining the "curtains" of the auroral arcs follow magnetic field lines.

16. Magnetic Storms and Ring Currents

The method used by Gauss and his followers to analyze the observed magnetic field showed that at least 99% of it originated inside the Earth. But external sources also existed: magnetic storms (typically changing the field by 1% or less) were world-wide phenomena and seemed to indicate that a powerful electric current was established around the Earth's equator, flowing at an undetermined distance for many hours or even days. Arthur Schuster in 1911 gave it the name we still use, the ring current [Smith 1963].

The polar aurora (sometimes known as aurora borealis or northern lights) also seemed to be linked to the magnetic field. Its dancing and glowing ribbons (usually a shade of green, but sometimes red) tend to maintain a constant distance from the magnetic pole of about 2-3000 kilometers (i.e. a constant "magnetic latitude"); near the magnetic pole itself they are rarely seen. In great magnetic storms they temporarily descend to lower (magnetic) latitudes and become visible at centers of population, more distant from the poles. At such locations the aurora is a rare phenomenon, but it isn't so for residents of central Alaska or Canada. Auroral ribbons often consist of many parallel rays (Figure 11), constantly fading or brightening, each aligned with the direction of local magnetic field lines. As found in 1741 by Hiorter and Celsius in Sweden [Beckman, 2000], bright displays are also accompanied by local disturbances in the Earth's magnetic field, often several times stronger than those of world-wide magnetic storms.

Fig.12 Kristian Birkeland with one of his terrella experiments.

What caused all this? Kristian Birkeland (1867-1917) in Norway thought (correctly) that the glow came from fast electrons hitting the high atmosphere, and tried to simulate the phenomenon on a small scale [see Stern, 1989]. Inside a glass vacuum chamber [Bruntland, 1998] he sent an electron beam towards a magnetized sphere representing the Earth, which like Gilbert he named terrella (Figure 12). He found that such beams tended to follow magnetic field lines, which guided them to the poles of a magnet or a terrella inside the chamber. The renowned French mathematician Henri Poincaré (1854-1912) then calculated that, at least with straight converging field lines, such electrons would in fact spiral along those lines and would be reflected from regions of more intense magnetic field, closer to the cone's apex.

Auroral outbursts were known to be associated with solar activity, and Birkeland guessed that they originated in beams of electrons emitted from the Sun. However, neither he nor his younger associate, the mathematician Carl Störmer (1874-1957), could explain why the aurora avoided the magnetic poles themselves, preferring instead to occur in large ring-shaped regions around them [Störmer, 1955].

Birkeland also observed magnetic disturbances associated with auroras, using a network of 4 ground stations, and concluded that they arose from electric currents flowing in the high atmosphere, along auroral arcs, linked at their ends to distant space, where the circuit somehow closed. The auroras and their associated currents occasionally intensified for an hour or so, and Birkeland termed such events polar magnetic storms. Unlike regular magnetic storms, these were localized and could not be observed at lower latitudes, where most magnetic observatories (as well as centers of population) were located.

The correlation between magnetic storms and activity on the Sun indeed suggested that something was being emitted by the Sun towards Earth. But not beams of electrons, as Birkeland had suggested, for it could be shown that electrostatic repulsion would disperse such beams long before they reached Earth. Instead, Sydney Chapman (1882-1970) and Vincent Ferraro (1907-1974) proposed in 1930 that the Sun emitted huge clouds of plasma, containing equal numbers of positive charges and electrons. Being electrically neutral, such a cloud could travel without dispersing.

However, it could not penetrate the Earth's magnetic field. To do so, its ions and electrons would have to attach themselves to terrestrial field lines and share them with terrestrial plasma, e.g. that of the ionosphere. By the theorem of field line preservation of the "freezing" of field lines (see section 13 on the solar dynamo), particles which initially did not share field lines cannot suddenly start doing so.

Field lines linked to the Earth's poles would therefore remain confined inside a "Chapman-Ferraro cavity" around which the cloud would wrap itself. The sudden confinement was expected to compress the Earth's field lines, and explained a small abrupt jump in magnetic intensity ("sudden commencement") observed on the surface at the start of many magnetic storms. Chapman and Ferraro speculated that somehow the cavity also generated the ring current, but how this happened they did not know.

Fig.13 Schematic view of the motion of an ion or electron trapped in the Earth's magnetic field. Not to scale: actually the orbit is much narrower near the Earth.

The ring current presumably consisted of charged particles trapped in the Earth's magnetic field. Störmer traced some of their motions by numerical calculations (in 1908 these had to be done by hand), but his work concentrated on particles with extremely high energies, not like those of the aurora. S. Fred Singer however showed [Singer, 1957] that low energy particles, of either sign, could also do the job, provided their numbers were large enough. Such particles spiraled around field lines while sliding along them, but the sliding motion would stop and reverse (as Poincaré had shown) whenever the particle approached regions of more intense magnetic field. In the field of the Earth, the magnetic intensity increased as one approached the "feet" of the field line in the atmosphere, since these were the points (on any given field line) closest to the center of the Earth. Many trapped particles would therefore be reflected before reaching the dense atmosphere, and could stay trapped for a long time, bouncing back and forth across the equator (Figure 13).

Singer noted a slow secondary mechanism that would meanwhile move trapped particles from their guiding field lines, attaching them to neighboring ones and gradually transporting them all the way around the Earth (see fig. 4, [Stern, 1989] ). Electrons would be transported in one direction, ions in the opposite one--and in either case, the electric current they carried around the Earth was in the direction required by the ring current.

17. The Magnetosphere

(Relatively few references will be cited from here on, because more than 350 are given by Stern [1996], also available on the World Wide Web (http://www.phy6.org/Education/bh2_1.html ). See also Stern and Ness, [1982].)

Sputnik 1, launched on October 4, 1957, was the first artificial satellite of Earth. It was followed by several thousand others, many of them carrying scientific instruments. Scientific satellites soon confirmed some earlier guesses, disproved or modified others, and added unexpected features to the picture.

The satellites did find belts of trapped ions and electrons, moving in the way proposed by Singer--but they were a permanent feature, not a temporary one present only during magnetic storms [e.g. Stern, 1996; Stern and Ness, 1982]. The first US satellites, Explorers 1 and 3 built by James Van Allen and his team at the University of Iowa, were launched in early 1958 and discovered an intense belt of trapped fast protons, above the magnetic equator. This "inner radiation belt" turned out to be a secondary product of the cosmic radiation, which is a population of extremely energetic protons believed to fill our galaxy. Cosmic rays provide a rather weak source, but the trapping is quite stable, allowing the protons to accumulate over a long time.

The inner belt, peaking at distances between 1.3 and 2 RE (earth radii) from the Earth's center, was dense enough to constitute a radiation hazard (astronauts can cross it safely, but only if they do not linger), however it was not extensive enough to carry the ring current. That was done by the outer radiation belt (2-8 RE), found to contain protons and electrons of moderate energies but in much greater numbers. Magnetic storms greatly increased its population, and with it also the intensity of the ring current, though some of the outer belt and of the ring current existed at all times.

The Sun indeed emitted plasma which flowed out radially at great speed, but again, it did so at all times, not just during magnetic storms as Chapman and Ferraro had assumed. That was the solar wind, predicted in 1958 by Eugene Parker, after he tried to calculate the equilibrium structure of the Sun's corona. The calculation indicated that an extremely hot plasma like that of Sun's corona could not be held in gravitational equilibrium, the way the Earth's atmosphere was. Instead, Parker's calculation suggested that the Sun continually sloughed off plasma in a supersonic stream, expanding radially in all direction. Though Parker's prediction was at first controversial, spacecraft instruments soon confirmed it. In 1961 its flow was measured by a "Faraday cup" placed aboard Explorer 10 by Herb Bridge and Bruno Rossi (1905-93), and more extensively by the space probe Mariner 2 in 1962.

Fig.14 Magnetic field lines and the bow shock (not a field line) in the Earth's magnetosphere, with some named features (not to scale).

By then Explorer 12 had crossed a well defined boundary on the sun-facing side of the "Chapman-Ferraro cavity" (renamed by Tom Gold "magnetosphere"), suggesting that the cavity also existed at all times. The boundary was named "magnetopause." The "sudden commencement" jumps of the magnetic intensity, at the onset of many magnetic storms, turned out to mark the arrival of fast plasma clouds, which plowed through the ordinary solar wind and created shock fronts ahead of themselves.
One feature which no one had expected before the satellite era was the long magnetic tail behind the Earth. On the sunward side the solar wind compressed the magnetospheric cavity, creating an abrupt boundary at an average distance of about 10.5 RE, though some interplanetary clouds, associated with magnetic storms, could push it to distances of 6 RE and even less. On the night side, on the other hand, the Earth's field lines were stretched out in two great bundles, each linked to one of the polar caps (Figure 14). Later spacecraft would observe these bundles at distances of more than 3 times that of the Moon, 200 RE from Earth and beyond.

Wedged between the two bundles--the two "tail lobes"--was the tail's plasma sheet, a layer of hot plasma which turned out to be the main source of auroral electrons. The magnetic field of the plasma sheet is weak, with hairpin-shaped magnetic field lines, and that turned it into a somewhat unstable region, the origin of many of the magnetic disturbances observed in space and on the ground. The reason auroras are rare near the magnetic poles, but occur fairly regularly along large rings surrounding each pole, is that auroral electrons (and their associated electric currents) are guided along magnetic field lines. Magnetic field lines from the "auroral oval" lead back to the plasma sheet, whereas those from the magnetic poles themselves (and the regions near them) end up in the twin bundles of the tail lobes, which contain rather little plasma.

Birkeland's observations were also validated, again with some modifications. What he called polar magnetic storms are now known as magnetic substorms, so named by Chapman who at first viewed them as component parts of magnetic storms--before it was realized that they occurred at other times as well. They represent violent changes in the plasma sheet which energize its ions and electrons, hurl them earthward, and in that way contribute to the outer ring current. Magnetic storms, following the arrival of a fast cloud, create very powerful disturbances of this kind and seem to be the main agent replenishing the ring current, though the details still need to be worked out.

The electric current system, which Birkeland claimed accompanied the aurora, was also observed, though its structure was somewhat different from what Birkeland had proposed. It forms an intricate pattern first traced by Alfred Zmuda and James Armstrong, using a "piggyback" (free ride) magnetometer aboard the Navy satellite Triad. Milo Schield, Alex Dessler and John Freemen, who theoretically predicted such currents in 1969, named them "Birkeland currents" and the name is still used. Birkeland currents provide the energy for most auroral arcs, but auroral electrons are then energized only close to Earth, getting their main push in the lowest 1-1.5 RE of their guiding field lines.

18. Magnetic Reconnection

Fig 15. Dungey's view of plasma flow and magnetic field lines at an X-type neutral point.

In such a brief description it is hard to do justice to the extensive field of magnetospheric physics. Like most physical processes, those of the magnetosphere, too, must usually be paid for in energy, the universal currency of physics. Except for the inner belt, the source of that energy seems to be the solar wind. If the separation between interplanetary and terrestrial field lines were strictly enforced, it would be rather difficult to transmit energy from one region to the other, but there exists a loophole. Magnetic field lines can change their linkage and "reconnect" in new ways, if the plasma in which they are embedded flows through an "x-type neutral point" (or "neutral line") at which the field intensity drops to zero and field lines cross in the pattern of the letter x (Figure 15).

James Dungey in England proposed [Dungey, 1961; Stern, 1986] that two such points were formed on the magnetopause, the boundary of the magnetosphere. One occured on the sunward side, where interplanetary magnetic field lines and terrestrial lines formed an x-shaped neutral point (or a continuous line of such points), at which they split up and reconnected. If the interplanetary field points exactly south, the plasma flows along the thick arrows in Figure 15--the solar wind plasma arriving from the left and the magnetospheric plasma coming from the right. The northern half of the terrestrial line then connects to the northern half of the interplanetary line, a similar process joins the southern halves, and the plasmas attached to the newly reconnected lines then flow outwards--up or down, in Figure 15.

This, Dungey assumed, created "open" field lines, starting in the polar regions of Earth and extending to interplanetary space, and along such lines, energy and plasma easily flowed from the solar wind to the magnetosphere. The process ended (in Dungey's original theory) at a second neutral point in the distant tail, where reconnection re-united the two terrestrial halves and the two interplanetary halves, after which the two plasmas were again separated.

If the interplanetary field lines pointed not southwards but just slanted towards the south, reconnection is a bit more complicated, introducing a bend in each "open" field line, at which the direction of the terrestrial line gradually changes over to that of the interplanetary one. If the field line slants northwards, the bending is more severe--the northern half of the terrestrial field line must link up with the interplanetary half which comes from the south, and vice versa. If the northward slant is steep, reconnection is expected to become increasingly difficult--the interplanetary field lines just have the "wrong direction" for linking up.

The actual process is probably much more complicated, but one of its predictions was amply confirmed. It was found [Fairfield and Cahill, 1966] that the most important factor promoting magnetic substorms, large Birkeland currents and other active phenomena was the slant of the interplanetary magnetic field. With southward slant activity was likely, with northward slant it was inhibited, and other factors--such as the velocity and density of the solar wind--were much less important.

Magnetic reconnection may also occur in the plasma sheet, constituting there an important element of the substorm process, but details will have to wait for more thorough studies of the magnetosphere, using simultaneous data from a much larger number of satellites than is available now.

Reconnection, acceleration and energization of plasma in the magnetosphere all involve a complicated interplay between magnetic and electric fields. Static magnetic fields can trap and steer electrons and ions without changing their energy. Since the fields do not have to supply energy, they can maintain the trapping indefinitely without requiring any additional energy input.

On the other hand, an electric field, a region of electric forces, is not only able to energize particles, it can also help them move from one field line to another, e.g. enter regions of magnetic trapping or escape from them. Electric fields therefore have a central role, both in bringing fresh particles into the ring current, as happens during magnetic storms, and in adding energy to those particles.

In addition, electric forces parallel to magnetic field lines are also instrumental in accelerating (negative) auroral electrons downward. By its nature, any "parallel electric field" which does so, will also grab positive oxygen ions from the ionosphere and accelerate them upwards, towards the magnetic equator, where many join the ring current; such "ion beams" were discovered in 1977 by the USAF satellite S3-3. Furthermore, electric fields are also essential to the energy exchange in substorms and to reconnection. The details of such processes (to the extent they are understood) are, however, completely beyond the scope of this brief and non-mathematical overview.